B. M. BINDON

When ewes are injected with synthetic Gn-RH on the day of oestrus, the magnitude of the LH release is greater than that following injection at other stages of the oestrous cycle (Reeves, Arimura & Schally, 1971; Hooley, Baxter, Chamley, Cumming & Findlay, 1974; Thimonier, Terqui & Pelletier, 1974). It is not known, however, whether additional LH on the day of oestrus has any effect on the number of ovulations, especially in ewes with the genetic potential for high ovulation rates. This question was investigated in four flocks of Merino sheep, designated `C', `O', `T' and `B', which are, respectively, an unselected group and groups selected since 1954, for low, medium and high litter size. The genetic history of these flocks has been described by Turner (1969).

B. M. BINDON

It is thought that unfertilized mammalian ova degenerate and fragment after ovulation and are soon absorbed either in the oviducts or uterus (Blandau, 1961). The results presented here indicate that, in the sheep, the unfertilized ovum, with zona pellucida intact, is retained in the uterus until at least 15 days after ovulation.
Groups of adult Merino and crossbred ewes, weighing 45 to 50 kg, were killed on Days 5, 6,7, 9, 11, 12, 13 or 15 of the oestrous cycle. Day 1 was the day on which ewes were served by a vasectomized ram. After slaughter, the reproductive tract was removed by severing at the caudal end of the cervix, the number of active corpora lutea in the ovaries recorded and the uterus prepared for flushing. Each uterine horn was flushed with 5 ml of 0·9% NaCl by means of a hypodermic syringe introduced at a point just caudal

B. M. Bindon

B. M. BINDON

B. J. RESTALL and B. M. BINDON

Summary.

Spacing of zygotes, loss of the zona pellucida, and appearance of localized areas of increased vascular permeability at the implantation sites in the uterus were observed in groups of mice examined at 2-hr intervals from 10.00 hours to midnight on Day 4 of pregnancy, and at 02.00, 08.30 and 15.00 hours on Day 5. All the events examined had a 6- to 8-hr variation in their time of occurrence.

Studies of interrelationships between the zygote and the uterus require a precise knowledge of the timing, variation and sequence of the events leading up to implantation in any one species. Previous studies of preimplantation phenomena in the rodent have been based on relatively small numbers of animals examined at irregular intervals during the periods of interest.

E. M. ROBERTS and B. M. BINDON

Summary.

One hundred and four anoestrous Merino ewes and 120 Romney Marsh ewes were treated in separate experiments of factorial design with combinations of a single injection of 6-chloro Δ6-17-acetoxyprogesterone (cap), followed some days later by the oral administration of 6-methyl-17-α-acetoxyprogesterone (map). An injection of pregnant mare's serum (pms) was then given at two different times in relation to the final map treatment.

Dose of cap, number of map treatments and time of pms injection all influenced the proportion of ewes exhibiting oestrus.

In the first experiment, twenty-six ewes lambed of the twenty-seven ewes served, in the second experiment only four ewes lambed of the twenty-four served.

B. M. BINDON and PAMELA R. PENNYCUIK

It is of interest to know whether genetic differences in fecundity arise as a result of changes in the sensitivity of the ovary to gonadotrophin or of increased secretion of gonadotrophin. This has a bearing on whether efforts to alter fecundity by genetic means should be concentrated at the level of the ovary, the pituitary gland, or both.

There are a number of studies in mice in which genetic differences in ovulation rate or litter size have been attributed to changes in ovarian sensitivity (McLaren, 1962; Land & Falconer, 1969; Wolfe, 1971). This conclusion has generally been based on the ovulatory response to exogenous gonadotrophin by intact mice of different genetic groups. Such tests, however, are influenced by the endogenous gonadotrophin levels of the intact test animal (Lamond & Emmens, 1959; Lamond & Bindon,

B. M. BINDON and D. R. LAMOND

Summary.

Experiments were conducted in pregnant mice in an attempt to clarify the role of the pituitary gland in implantation in this species. Hypophysectomy was performed at various times during Days 2 to 5 of pregnancy and replacement studies with human chorionic gonadotrophin (hcg) or progesterone were carried out.

Results indicated that pituitary involvement in the initiation of implantation was confined to the early part of Day 3. Changes in the photoperiod altered the time of pituitary involvement, thus exposing a possible source of error in between-laboratory comparisons in this type of study.

Pregnancy was maintained in hypophysectomized mice by daily injection with hcg. The minimum daily dose was between 1·1 and 3·3 i.u. Embryos maintained by hcg were smaller than those in hypophysectomized mice injected with progesterone.

B. M. BINDON and D. R. LAMOND

Summary.

The uterine weight and ovulation responses of immature intact and hypophysectomized mice from two lighting regimes have been studied after injection of gonadotrophin. The results indicate diurnal variation in ovarian response: the response to gonadotrophin was greater when it was administered during the second half of the light phase of the daily light cycle than at any other time.

Pregnant mice were fasted for 36- to 48-hr periods beginning at 06.00 hours or 18.00 hours on the 2nd, 3rd or 4th day after mating. Diurnal variation in the proportion of pregnancies failing was demonstrated. The effect was not attributable to differences in loss of body weight during fasting, and could be prevented by injections of human chorionic gonadotrophin (hcg).

The numbers of eggs ovulated by adult di-oestrous mice treated with hcg were studied after 36- or 48-hr fasts commencing at 06.00 hours or 18.00 hours 2 days before an injection of hcg. Fasting reduced numbers of ovulations, but the effect was influenced by the time of day when the fast began. Reversal of the lighting regime resulted in a reversal of the pattern.